A synchronization mechanism is incorporated in a manual transmission so that a drive shaft rotates in synchronism with a transmission gear at the time of a gear shift operation. A synchronizer ring is incorporated in such a synchronization mechanism. As an example of the synchronization mechanism incorporating such a synchronizer ring 1. In a synchronization mechanism, an insert spring 4 is mounted on an outer circumference of a synchronizing hub 3 which rotates together with a drive shaft 2, and a coupling sleeve 5 displaced in an axial direction on the basis of an operation of a shift lever is engaged with the insert spring 4. An outer balk ring 8, a synchronizer ring 1, and an inner balk ring 9 are provided between the coupling sleeve 5 and a clutch gear 7 which rotates in synchronism with a transmission gear 6.
When the coupling sleeve 5 is pushed to the left in FIG. 8 in accordance with a gear shift operation, first, a rotational speed difference between the coupling sleeve 5 and the clutch gear 7 is eliminated by a frictional force acting between an outer balk ring 8 and the synchronizer ring 1 and between the synchronizer ring 1 and an inner balk ring 9. When the coupling sleeve 5 is further pushed from this state, a spline groove 10 formed on an inner circumferential surface of the coupling sleeve 5 engages with a spline groove 11 formed on an outer circumferential edge of the clutch gear 7 and a spline groove 12 formed on an outer circumferential edge of the insert spring 4 so as to stretch over them, and thus the drive shaft 2 rotates in synchronism with the transmission gear 6.
The synchronizer ring 1 incorporated in the synchronization mechanism configured and operating as described above is configured in a shape, for example, as illustrated in FIGS. 9 and 10. That is, the synchronizer ring 1 is constituted by a tapered cylindrical portion 13 of which a diameter changes in the axial direction, and claw pieces 15 and 15 formed at regular intervals at a plurality of positions on a large diameter side end surface 14 of the tapered cylindrical portion 13. A thickness dimension t of each of the claw pieces 15 and 15 is smaller than a thickness dimension T of the tapered cylindrical portion 13 (T>t). Further, a direction of each of the claw pieces 15 and 15 is different depending on a structure of synchronization mechanisms into which the synchronizer ring 1 is to be incorporated, and may be inclined in the same direction as the tapered cylindrical portion 13, or the claw pieces 15 and 15 may be parallel to each other.
As a method of manufacturing the synchronizer ring 1 having such a shape, a method of manufacturing the synchronizer ring 1 at low cost by subjecting a flat metal plate to press processing is described in Patent Document 1. FIGS. 11 to 15 illustrate a manufacturing method of a synchronizer ring described in Patent Document 1 and conventionally known. First, a first ring-shaped workpiece 16 as illustrated in FIGS. 12 and 13 is formed by so-called blanking which punches out a flat metal plate such as carburized steel such as SCr420. The first ring-shaped workpiece 16 includes an annular main body portion 17 and tongue-shaped portions 18 and 18 protruding from a plurality of positions on an outer circumferential edge of the main body portion 17. Next, a second ring-shaped workpiece 19 as illustrated in FIGS. 14 and 15 is formed by press-molding the first ring-shaped workpiece 16. That is, in the molding step, the main body portion 17 (FIGS. 12 and 13) is formed into a tapered cylindrical portion 20 inclined in a direction in which a diameter thereof decreases with distance away from the tongue-shaped portions 18 and 18 so that the second ring-shaped workpiece 19 is formed. Next, an end surface of the tapered cylindrical portion 20 and each of the tongue-shaped portions 18 and 18 which constitute the second ring-shaped workpiece 19 are subjected to machining such as turning so that the tapered cylindrical portion 20 and each of the tongue-shaped portions 18 and 18 have predetermined shapes and dimensions. Next, after burrs generated on the basis of the machining are removed, heat treatment is performed to obtain a desired hardness. In the heat treatment, for example, a carburizing heat treatment is performed, and thereafter, deformation based on heat treatment is corrected. A third ring-shaped workpiece on which such a heat treatment is performed as above is subjected to surface grinding for finishing flat surfaces of the respective tongue-shaped portions 18 and 18 to form the respective claw pieces 15 and 15 as well as inner and outer diameter grinding for smoothing both the inner and outer circumferential surfaces of the tapered cylindrical portion 20 to form the tapered cylindrical portion 13 so that the synchronizer ring 1 is formed. Then, the synchronizer ring 1 manufactured in this manner is shipped after performing a predetermined inspection.
Incidentally, in the case of the method of manufacturing the synchronizer ring 1 described in Patent Document 1 as described above, one synchronizer ring 1 is formed from the first ring-shaped workpiece 16 formed by applying a punching process to a flat metal plate. Therefore, there is room for improvement from a manufacturing efficiency perspective.